Surface contour imaging uses patterned or structured light and triangulation to obtain surface contour information for an object. In contour imaging, a pattern of lines or other features is projected toward the surface of an object from a given angle. The projected pattern on the surface is then viewed from another angle as a contour image, taking advantage of triangulation in order to analyze surface information and to characterize the surface contour based on the deformed appearance of the projected lines. Phase shifting, in which the projected line pattern is incrementally spatially shifted for obtaining additional measurements at higher resolution, helps to more accurately map the object's surface.
Surface contour imaging using structured light has been employed in a number of applications for determining the shape of solid, highly opaque objects. Contour imaging has also been used for characterizing the surface shape of portions of the anatomy and for obtaining detailed data about skin structure. However, a number of technical obstacles complicate effective use of contour projection imaging of the tooth. One particular challenge with dental surface imaging relates to tooth translucency. Translucent or semi-translucent materials in general are known to be particularly troublesome for patterned light imaging. Subsurface scattering in translucent structures can reduce the overall signal-to-noise (S/N) ratio and shift the light intensity, causing inaccurate height data. Another problem relates to high levels of reflection for various tooth surfaces. Highly reflective materials, particularly hollowed reflective structures, can effectively reduce the dynamic range of this type of imaging.
From an optical perspective, the structure of the tooth itself presents a number of additional challenges for structured light projection imaging. Teeth can be wet or dry at different times and along different surfaces and portions of surfaces. Tooth shape is often irregular, with sharp edges. As noted earlier, teeth interact with light in a complex manner. Light penetrating beneath the surface of the tooth tends to undergo significant scattering within the translucent tooth material. Moreover, reflection from opaque features beneath the tooth surface can also occur, adding noise that degrades the sensed signal and thus further complicates the task of tooth surface analysis. Not all light wavelengths can be detected with equal accuracy. Thus, a multi-spectral or multicolor approach can be less satisfactory in some cases.
Even where a coating or other type of surface conditioning of the tooth is used, however, results can be disappointing due to the pronounced contours of the tooth surface and inherent difficulties such as angular and space limitations. It can be difficult to provide sufficient amounts of light onto, and sense light reflected back from, all of the tooth surfaces. For example, different surfaces of the same tooth can be oriented at 90 degrees relative to each other, making it difficult to direct enough light for accurately imaging all parts of the tooth.
There have been a number of attempts to adapt structured light surface-profiling techniques to the problems of tooth structure imaging. For example, U.S. Pat. No. 5,372,502 entitled “Optical Probe and Method for the Three-Dimensional Surveying of Teeth” to Massen et al. describes the use of an LCD matrix to form patterns of stripes for projection onto the tooth surface. A similar approach is described in U.S. Patent Application Publication 2007/0086762 entitled “Front End for 3-D Imaging Camera” by O'Keefe et al. U.S. Pat. No. 7,312,924 entitled “Polarizing Multiplexer and Methods for Intra-Oral Scanning” to Trissel describes a method for profiling the tooth surface using triangularization and polarized light, but requiring application of a fluorescent coating for operation. Similarly, U.S. Pat. No. 6,885,464 entitled “3-D Camera for Recording Surface Structures, in Particular for Dental Purposes” to Pfeiffer et al. discloses a dental imaging apparatus using triangularization but also requiring the application of an opaque powder to the tooth surface for imaging. U.S. Pat. No. 6,885,464 to Pfeiffer et al. describes an intraoral camera that provides a group of light beams for imaging. Patent application WO 2011/145799 by Lim describes a 3-D scanner using scanned laser light. Patent application WO 2016/041147 by Liu describes a laser projection apparatus for contour imaging using a line laser.
Reference is hereby made to an article by Hiroshi Aoki entitled “Development of portable 3D optical measuring system using structured light projection method” in Proceedings of SPIE, vol. 9110 (2014) pp. 91100Q-1 to 91100Q-8; to an article by Tom Yoshizawa entitled “Compact Camera for Real-Time 3D Measurement” 10th IMEKO TC14 Symposium on Laser Metrology for Precision Measurement and Inspection in Industry (September 2011) pp. 1-6; and to an article by Yasuhiro Takaki entitled “Micromirrors and 1D scanning produce an enlarged holographic color display” in SPIE Newsroom (2014) pp. 1-3.
Conventional methods for forming a pattern of lines include use of a 2-D array of micromirrors, such as those provided by a Digital Light Processor (DLP) from Texas Instruments, Inc., Dallas, Tex. Designs proposed for using these devices, however, are bulky and poorly suited for applications such as intraoral imaging. Alternate solutions using 2-D scanners have been proposed; however, these solutions do not allow imaging at the needed speed for intraoral applications.
Thus, it can be appreciated that there would be benefits to an optical apparatus for intraoral surface contour imaging that is highly compact, lightweight, and provides sufficient scan speed for practical use in intraoral imaging applications.